Electrophotographic process and apparatus

ABSTRACT

LIGHT WHILE THE INSULATIVE OVERLAYER IS SUBJECTED TO A.C. CORONA DISCHARGE WHEREBY SAID INTERFACE CHARGE IS SELECTIVELY ATTENUATED TO FORM SAID ELECTROSTATIC IMAGE. THE FORMED IMAGE IS INTENSIFIED IN CONTRAST BY THEREAFTER EXPOSING THE PHOTOCONDUCTIVE LAYER TO RADIATION.   A PROCESS FOR FORMING AN ELECTROSTATIC IMAGE IN A PLATE HAVING A PHOTOCONDUCTIVE LAYER WITH AN INSULATIVE OVERLAYER AND BEING CHARACTERIZED IN HAVING CARRIER CHARGE OF A POLARITY CORRESPONDING TO THE CONDUCTIVITY TYPE OF THE PHOTOCONDUCTIVE LAYER INJECTABLE INTO THE PHOTOCONDUCTIVE LAYER AND BOUND IN THE REGION OF TH INTERFACE BETWEEN THE INSULATIVE AND PHOTOCONDUCTIVE LAYER, WHEREIN THE INSULATIVE LAYER IS INITIALLY CHARGED WITH A POLARITY INDUCING SEMICONDUCTION IN THE PHOTOCINDUCTIVE LAYER AND CAUSING CHARGE MIGRATION THEREIN AND THERETHROUGH TO THE INTERFACE OF THE INSULATIVE AND PHOTOCONDUCTIVE LAYERS. THE PHOTOCONDUCTIVE LAYER IS NEXT EXPOSED TO PATTERN OF IMAGE

30, 1972 HIROSHI TANAKA ETAL 3,666,363

ELECTROPHOTOGRAPHIC PROCESS AND APPARATUS Filed Feb. 18, 1971 2 Sheets-Sheet 1 1 w m INVbNTORs T v Measw 75/4/(4 law-sum YAGAM? 71w BY Gucm MflRMSW/MA SHINKICH/ firm/Maw EYS May 30, 1972 HlROSH] TANAKA ETAL 3,666,363

ELECTROPHOTOGRAPHIC PROCESS AND APPARATUS Filed Feb. 18, 1971 2 Sheets-Sheet 2 a m w w W IX M MMMMW m MWMWW A wmmm M3 M3 V! B United States Patent 01 3,666,363 Patented May 30, 1972 3,666,363 ELECTROPHOTOGRAPHIC PROCESS AND APPARATUS Hiroshi Tanaka, Katsumi Nagamatsu, Giichi Marushima,

U.S. Cl. 355-17 1 Claims ABSTRACT OF THE DISCLOSURE A process for forming an electrostatic image in a plate having a photoconductive layer with an insulative overlayer and being characterized in having carrier charge of a polarity corresponding to the conductivity type of the photoconductive layer injectable into the photoconductive layer and bound in the region of the interface between the insulative and photoconductive layer, wherein the insulative layer is initially charged with a polarity inducing semiconduction in the photoconductive layer and causing charge migration therein and therethrough to the interface of the insulative and photoconductive layers. The photoconductive layer is next exposed to a pattern of image light while the insulative overlayer is subjected to A.C. corona discharge whereby said interface charge is selectively attenuated to form said electrostatic image. The formed image is intensified in contrast by thereafter exposing the photoconductive layer to radiation.

This is a continuation of U.S. patent application Ser. No. 571,538, filed Aug. 10, 1966, now abandoned.

The present invention relates to electrophotography in general, and in particular, this invention relates to methords and apparatus for forming electrostatic and electrophotographic images.

Known electrophotographic methods include such conventional methods as the Electro Fax system, the Xerox system, the P.I.P. (Persistent Internal Polarization) system and the like. The Electro Fax and Xerox systems form electrostatic images by means of the so-called Carlson process as described in the specification of U.S. Pat. No. 2,297,691. According to these systems, the photoconductive layer of a photosensitive plate comprised of zinc oxide (Electro Fax), or non-crystalline selenium (Xerox) disposed on a base plate, is uniformly charged by corona discharge, and thereafter is irradiated with an original image to impart charge to the illuminated portion to form an electrostatic image in accordance with the light-anddark pattern of the original image. The electrostatic image is developed by using electroscopic powder (hereinafter called toner) to form a visual image, and then the said image is fixed (Electro Fax) or transferred onto a support such as paper and thereafter it is fixed (Xerox) to obtain an electrophotographic image. In accordance with P.I.P. system, the photosensitive plate comprises a mixture of phosphor and resin disposed on a conductive base plate and is pinched with two electrodes. A voltage is applied to the two electrodes to generate persistent internal polarizing charge in the photoconductive layer, and then by irradiating the plate with an original image, an electrostatic image is obtained by persistent internal polarizing charge in accordance with the light-and dark pattern of the original image. Thereafter development and fixing processes are carried out in the same manner as in the abovementioned cases, and an electrophotographic image is obtained.

In the above-mentioned systems, it is necessary to retain charge directly in the photoconductive layer, and therefore it is required that the material used for forming the said photoconductive layer should be of high resistivity, and for example, be restricted to specific photoconductive materials which can bind charge and which have high resistivity, such as non-crystalline selenium, Zn0+resin, Zn CdS+resin or the like.

Therefore, machines in present practical use have low sensitivity, and in the case of Electro Fax, the sensitivity is below ASA 5 even if acceleration of sensitivity should be carried out by using dyes, and even in the case of Xerox system, or P.I.P. system, the sensitivity is ASA 10 at maximum.

When the above-mentioned photosensitive plate is repeatedly used, scars on the surface, or other deterioration of the surface easily occurs, and the quality of the image deteriorates because of fatigue of the photoconductive material. Thus, such plates cannot withstand repeated use over long periods.

There has been proposed a further method described in the specification of U.S. Pat. No. 3,124,456 according to which a photosensitive plate having a photoconductive layer composed of CdS or CdSe and binder resin on a conductive base is provided with a translucent insulating layer overlaid on the photoconductive layer. Irradiation of the original image and charging are carried out simultaneously from the translucent insulating layer side of the photosensitive plate and the electrostatic image is formed on the translucent insulating layer by making use of the difference of the buildup of the charge due to the difference of time constants caused by the different impedances of the photoconductive layer in the light and dark portions, respectively, of the original image. In accordance with this method, electrostatic image formation depends on the time constant dilference brought about by the diiferences of impedance in the photoconductor and therefore the electrostatic contrast is not high. To obtain an excellent image by means of this method, the capacitance of the translucent insulating layer must be larger than the capacitance of the photoconductive layer, and form a practical point of view the thickness of the translucent insulating layer should be restricted within the range from 2 to 6/1..

Such thin insulating layers break down easily, and it is difficult to use the photosensitive plate over and over again for a long period of time.

Thus, in processes in which electrostatic image formation depends on the change of impedance of the photoconductive layer, where the thickness of translucent insulating layer is increased, contrast is deteriorated, and image quality is lowered, which are drawbacks.

On the other hand, according to U.S. Pat. No. 3,041,- 167 issued to R. M. Blakney et al., a photoconductive layer is provided on a conductive base plate, and the photosensitive plate is obtained by protecting the surface thereof with an overcoating layer. In accordance with the Carlson process, an electrostatic image is formed by such electrophotography as mentioned above. However, a reverse charge of opposite polarity to that of the sensitizing charge is applied to the surface of the coating layer of the said photosensitive plate before the said sensitizing charge is carried out. After sensitizing charging, light is uniformly irradiated all over the surface of the coating layer of the said photosensitive plate; this aims at overcoming the fatigue of the photosensitive plate. Photo-carriers produced by the whole surface exposure and having a polarity opposite to that of the charge of the insulating layer are induced at the interface between the insulating layer and the photoconductive layer. Then the sensitizing charge is applied in darkness to the surface of the insulating layer to neutralize the charge to form a surface field due to the induced charged layer. Thus the photosensitive plate is sensitized. Next, the light image is exposed to attenuate said inducedcharge and the latent image is formed by the charge remaining at the dark areas. This process is a Carlson-type process and the obtained contrast is as much as 300 to 500 volts.

In the photosensitive plate of the above-mentioned process, it is necessary that the overcoating layer be thin compared to the photoconductive layer. Therefore it is easy to bring about wear or breakdown or such like troubles and it is impossible to sufiiciently protect the photoconductive layer.

In accordance with the present invention, quite different from the above-mentioned process, the translucent i11- sulating layer is charged, and by making use of the field thereof, charge is strongly bound on the photoconductive layer and translucent insulating layer and in the neighborhood thereof, and by making use of the external field of the said bound charge, alternating current corona discharge and irradiation of the original image are simultaneously carried out, and an electrostatic image is obtained due to the difference of the surface potential of the translucent insulating layer at the light portion and the dark portion of the original image, and a further electrostatic image is obtained by reversing the said surface potential by irradiating light uniformly all over the surface of the said translucent insulating layer. Therefore, the electrostatic contrast is remarkably high, and even when a photoconductive layer of a little thicker than or the same thickness as that of U.S. Pat. No. 3,041,167 is used, it is possible to obtain electrostatic contrast ranging from 1000 v. to 1500 v., and thus it is possible to use a translucent insulating layer of to 50p thickness. Further, it is possible to form said layer by the adhesion of insulating film without being limited to the method of resin coating, and thereby it is possible to protect the photoconductive layer sufficiently.

In copending U.S. application No. 563,899, filed July 8, 1966, an electrostatic image is formed in a photosensitive plate comprising a base, a photoconductive layer and a translucent insulating layer, by applying a primary charge (assuming positive charge) to the translucent insulating layer, exposing the photoconductive layer to the original image while applying secondary charge (assuming negative charge) to the translucent insulating layer, and thereafter exposing the whole surface of the translucent insulating layer to form thereon an electrostatic image. The electrostatic image is mainly produced at light areas of the original image by the binding of the secondary charge, i.e. negative charge. Therefore, it is necessary, in obtaining a positive-positive visual image, to attach negatively charged toner on the dark area of the original image, or to apply positively charged toner to the light area of the original image to obtain positive-negative visual image. It follows therefore that when processing an original image having a large area or thick lines, in positive-positive development only the peripheral portions are emphasized and it is difficult to obtain a clear development of the center portion. Further, as is generally known, in the developing process using Tribo Charge, Tribo Charge is decreased under high humidity circumstances, and the dielectric property of the toner is increased by absorption of humidity or attachment of water. Consequently, with a large negative potential at light area, toner is induced to attach thereto and cause a foggy image. Moreover, in particular, when the carrier of the developer is iron powder, the charge bound on the translucent insulating layer is discharged through the carrier to cause a foggy image.

On the contrary, in accordance with the present invention, an electrostatic image isformed in a similar photosensitive plate, by applying a primary charge (assuming positive polarity) to the translucent insulating layer, exposing the original image simultaneously with the discharging of the primary charge by alternating current corona discharge, and thereafter exposing the whole surface to obtain an electrostatic image. Consequently, the electrostatic image is formed mainly by retained primary charge (positive polarity) at the dark area of the original image. Therefore, to obtain a positive-positive image, it is only necessary to attract negatively charged toner by the positive charge retained at the dark area of the original image, which easily enables wide area developing.

In accordance with the present invention, even if either or both of toner and carrier becomes conductive to cause an inducing phenomenon, there is no problem at all since the developing is carried out by the charge bound at dark area, and it produces a high density image. Furthermore, even if a high frictional force is applied to the charged area, since toner is firmly attached thereon, there is no fear of leakage of the charge and it is difficult to produce a foggy image. Moreover, since the high voltage source of alternate current discharge does not require a rectifier, it is very economical in practice.

An object of the present invention is to provide anew electrophotographic process and apparatus which can be repeatedly carried out and used for a long period of time by overcoming the drawbacks of the above-mentioned conventional electrophotographic methods.

Another object of the present invention is to provide an electrostatic image forming process comprising charging in positive or in negative, in advance, the surface of the translucent insulating layer of a photosensitive plate comprising a photoconductive layer overlaid on a conductive base body, said translucent insulating layer being overlaid on the said photoconductive layer, by means of electrodes or corona discharge or the like; and then irradiating the original image on the said insulating layer, and during the same time applying alternating current corona discharge thereto, and thus forming an electrostatic image defined by the surface potential generated in accordance with the light-and-dark pattern of the original image on the said translucent insulating layer.

A further object of this invention is to provide an electrostatic image-forming process comprising forming a first electrostatic image as mentioned above, and thereafter irradiating light all over the translucent insulating layer to reverse the said surface potential, and thereby forming a second electrostatic image of the original image of high contrast on the surface of the said insulating layer.

Another object of the present invention is to provide an electrophotographic process which comprises visualizing the electrostatic image formed as mentioned above by using developer, transferring the thus obtained visualized image onto a transfer material, fixing the same to obtain an electrophotographic image of the original image, and repeatedly using the photosensitive plate by cleaning the surface of the insulating layer after the electrostatic image is transferred.

Another object of the present invention is to provide electrophotographic image forming process comprising visualizing the electrostatic image formed on the translucent insulating layer as mentioned above by using electroscopic developer, and then charging the translucent insulating layer containing the said visualized image with optional polarity, and overlaying a transferring material thereon to transfer the visualized irnage, fixing the transferred image, for example, by means of heat, to obtain an electrophotographic image, and after having carried out the transfer of the image, cleaning the surface of the said insulating layer to remove the remaining developer, and using the said photosensitive plate over and over agaln.

The above-mentioned objects and other numerous objects of the present invention, and a number of characteristics and effects of this invention will be easily and clearly understood from the explanations of the embodiments of this invention shown in the drawings, in which:

FIG. 1 is a diagram showing the fundamental structure of an electrophotographic plate to be used in the process for forming the electrostatic image of the present invention;

FIGS. 2 through 4 are diagrams showing processes for forming an electrostatic image on the translucent insulating layer of the electrophotographic plate of FIG. 1;

FIG. 5 shows the potential on the surface of the insulating layer of the electrophotographic plate obtainable by the processes of FIGS. 2 to 4;

FIG. 6 shows a visible image obtainable on the surface of the insulating layer of the plate;

FIG. 7 shows a process for transferring the visible image of FIG. 6 onto copy material; and

FIG. 8 is an electrophotographic copying device embodying a process of the present invention.

FIG. 1 is a diagram which shows the fundamental structure of an electrophotographic plate used in the process for forming the electrostatic image of the present invention, and in the diagram, 1 is a base, 2 is a photoconductive layer coated on the base by using a sprayer, or a coater, or wheeler or the like, and if necessary, it is possible to add a little amount of binder material such as resins and the like. 3 is an insulating (insulative) layer which is closely adhered on photoconductive layer 2. Thus, photosensitive plate A has three layers, i.e., base 1, photoconductive layer 2, and insulating layer 3. It is also possible to form a control layer such as to control the transfer of charge between base 1 and photoconductive layer 2, and it is also possible to add or independently provide a layer for binding charge on the surface of photoconductive layer or in the neighborhood of the surface.

Base 1 is formed of conductive material. As a conductive base, tin, copper, aluminum, or like metal conductors can be used, but in particular, aluminum foil adhered on paper is economical and it is very convenient when used by winding on a drum or the like.

As the material which composes photoconductive layer 2, CdS, CdSe, ZnO, Se, TiO Se-Te, and it is also possible to use any of the above-mentioned materials by directly coating same on the base, or in the form of mixture along with binder, or a mixture of more than two kinds of the above-mentioned materials.

Among these photoconductive materials, highly sensitive materials especially adapted for the present invention, are highly photoconductive materials such as CdS, Se--Te, or the like, and when these materials are used, it is possible to elevate sensitivity up to ASA 100. On the other hand, a photoconductive layer obtained by adding a little amount of ZnS in a photosensitive layer mainly composed of CdS, is highly sensitive, and provides an electrostatic image of high contrast and high sensitivity.

Mixtures of CdS and ZnS have hitherto been used (in P.I.P. system), but in order to elevate the difference of photopolarization and dark polarization and internal polarization characteristics, the ratio of CdS to ZnS is selected to be within the range of 4:6 to 3:7.

On the contrary, in the case of the present invention, the ratio of CdS and ZnS is preferably within the range of 50:1 to 1:1, and it is possible to make advantageous use of the characteristics of the highly sensitive CdS.

Further in accordance with the process of the present invention, use is made of the charge bound persistently on the photoconductive layer of a photosensitive plate including an overlying insulating layer on the photoconductive layer as is described hereinafter, and an electrostatic image is formed on the surface of the overlying insulating layer. Therefore, it is possible to use photoconductive materials of low resistivity, such as Se--Te, CdS or the like which have heretofore not been possible to use because of the necessity for the photoconductive layer itself to bind charge. At the same time, it is possible to use conventional photoconductive material even if not especially highly sensitive.

'Photoconductive papers incorporating zinc oxide dispersed in a resin, which have hitherto been adopted in the conventional Electro Fax system, are required to be white because they are used as copying papers by themselves, and therefore it is impossible to add much dye to sensitize same, and it has not been possible to elevate the sensitivity sufficiently.

However, in accordance with the method of the present invention, the photosensitive plate is itself not used as copying paper, but a visualized image is made on transfer paper, and therefore it is not necessary for the photosensitive plate to be white, which enables one to add remarkably larger amount of dyes when compared with the amount of dyes used in conventional methods.

Therefore, it is possible to use, in accordance with the method of the present invention, a several times more sensitive zinc oxide photoconductive layer compared with conventional method.

-In the method of the present invention, a photoconductive layer obtained by doping lithium into Zn'O presents excellent results.

As to the material which constitutes insulating layer 3, any material can be used which has high resistance against wear, high resistivity and capability of binding electrostatic charge, and translucency to activating radiation. Films of the following resins can be used, i.e., fluorine resin, polycarbonate resin, polyethylene resin, polyester resin, or the like. In .particular, fluorine resin has a specific property which makes it easy to carry out cleaning, and therefore as is explained hereinafter, it is a preferable material in carrying out the method of the present invention for using the photoconductive plate over and over again through developing, transferring, and cleaning processes.

FIGS. 2 through 4 show processes for forming an electrostatic image on the translucent insulating layer 3 of photosensitive plate A, and the image of charge on the photosensitive plate. Through the process steps of primary charge (FIG. 2), and of irradiation of the original image while applying A.C. corona discharge (FIG. 3), the electrostatic image, obtained by generation of surface potential in accordance with the light-and dark pattern of the original image as shown in FIG. 3, is formed on the surface of the translucent insulating layer.

Further, a reverse electrostatic image, wherein this surface potential is reversed as shown in FIG. 4, is formed on the surface of the insulating layer by uniformly exposing all of the surface of translucent insulating layer 3.

The potential on the surface of the insulating layer in these processes is shown in FIG. 5. First of all, in a dark (unexposed) area or light (exposed) area, the surface of translucent insulating layer 3 of photosensitive plate A is charged in a definite polarity, for example, positive by means of the conventional charging means such as corona discharger 5 (FIG. 2) connected to high voltage source 4, or an electrode roller (not shown).

With the surface of insulating layer 3 charged in the positive, insulating layer 3 works as a condenser, and a charge of polarity opposite to the charging polarity is accumulated between insulating layer 3 and photoconductive layer 2, and in the neighborhood thereof.

This charge is considered to be composed of either free carriers of photoconductive layer 2, photo carriers, or carriers injected from the side of conductive base 1, or a mixture of these carriers.

The accumulated carriers are bound strongly by the barrier of the photoconductor which composes photoconductive layer 2, and is a charge of opposite polarity to that on the surface of the insulating layer. In this state, there is no fear that the charge should escape in either a light or dark area for a long period of time, and furthermore, in a dark area, the carrier charge inside the insulating layer is persistent even if the charge on the surface of the insulating layer is discharged.

As is shown in FIG. 3, the light image is obtained on translucent insulating layer 3 by activating irradiation of original image 8, having light areas 6 and dark areas 7,

by means of a penetrating or reflecting system. At the same time, by means of alternating current corona discharge connected to high voltage alternating current electric source 9, alternating corona discharge is applied. to insulating layer 3. The first charging polarity in the above-discussed charging processes is determined by the property of the photoconductor. In the case of a photoconductive layer whose photoconductor is mainly composed of GdS activated by copper, or ZnO, or such like n-type photoconductors, it is preferable to charge in the positive, and in the case of the photoconductive layer mainly composed of p-type photoconductors such as amorphous selenium, Se-Te, it is preferable to charge in the negative. However, this is not critical, and even in the case wherein charge polarity is not as preferably stated above, it is possible to obtain an electrostatic image although contrast is somewhat deteriorated.

As the means for carrying out the alternating current corona discharge while irradiating the original image on insulating layer 3 of the photosensitive plate, it is preferable to irradiate the original image on the photosensitive plate through the use of an alternating current corona discharger having a shield plate whose upper portion is translucent or an optically open shield plate having no upper shield member. For example, as is shown in FIG. 3, when alternating current corona discharger 10, whose upper portion is optically open, is moved while charging the surface of insulating layer 3, the light image of the original image is irradiated on the surface of the insulating layer through the alternating current corona discharger. Of course, the alternating current corona discharger may be fixed, and the original image and the photosensitive plate may be moved relative thereto. At this time, the effective area of discharge of the alternating corona discharger should preferably constitute a slit exposure width.

When the irradiation of the original image and the secondary charging alternating current corona discharge are carried out, as in FIG. 3, in the light areas of the original image, the primary charge in the positive on the surface of insulating layer 3 is wholly or mostly discharged thereby. The amount of this discharge depends on the intensity and/or the time duration of the alternating current corona discharge. Photoconductive layer 2 is reduced in resistivity by irradiation of the original image, and becomes conductive, and by means of the primary charge, the negative charge bound at the interface of photoconductive layer 2 and insulating layer 3, or the portion of the interior of photoconductive layer 2 close to insulating layer 3 is freed, and in accordance with the reduction of the surface charge of insulating layer 3, it is reduced, and most of the charge is discharged into conductive base 1. Therefore, the surface potential of insulating layer 3 is reduced in accordance with the time duration of the alternating corona discharge, and the specific characteristic is presented as V in FIG. 5.

In the above case, when the alternating corona discharge voltage is sufiiciently higher, such as about 7 kv. and discharge time is sufiiciently long, it is possible to charge the original image light areas of the insulative layer more or less in the negative.

On the other hand, in the dark areas of the original image, the positive charge, formed on the surface of insulating layer 3 by means of the primary charge, is discharged by irradiation and alternating current corona discharge but as to the degree thereof, it is a little less than that in the light area. This is considered to be attributable to the fact that the negative charge bound between photoconductive layer 2 and insulating layer 3 or on the portion of photoconductive layer 2 in the neighborhood of insulating layer 3 by means of the primary charge, remains without being discharged even if the alternating current corona discharge is applied thereon because the resistivity of photoconductive layer 2 is high, and because of the said negative charge, the positive charge of the surface of insulating layer 3 is restrained, and therefore the degree of discharge is decreased.

As a result thereof, the surface potential in the dark areas of the original image is lower than the surface potential in the light area and the specific characteristic is presented as V in \FIG. 5.

When the alternating current corona discharge voltage is sufficiently elevated in comparison to that of the abovementioned case, for example, over 7 kv., and when the time for discharge is sufiiciently long, the surface charge of insulating layer 3 is readily neutralized, and sometimes the surface potential of the insulating layer is turned slightly negative by means of the field of the negative charge bound on photoconductive layer 2. On the surface of insulating layer 3 after the above-mentioned processes, the surface potential (V V difference is generated in accordance with the light-and-dark pattern of the original image, and the electrostatic image of the original image caused by this surface potential is formed.

The above-mentioned surface potential difference (V V is changed, as is shown in FIG. 5, according to the time of the alternating corona discharging and the irradiation of the original image, and therefore in order to obtain greater surface potential difference, it is necessary to appropriately select the time for irradiation of the original image and the time for carrying out corona discharge.

When the photoconductive layer of the photosensitive plate is thin, or when the bound charge is weak, the value of the surface potentials V and V become almost equal, and the surface potential difference (V -V is almost not observed. This can be considered due to the fact that the bound charge is weak within the photoconductive layer and it is comparatively quickly neutralized by means of the electric field.

Next, when light is irradiated onto the whole surface of insulating layer 3, after forming the electrostatic image on the surface of insulating layer 3 by carrying out the alternating current corona discharge and the irradiation of the original image for an appropriate period of time, there is no remarkable change in the state of the photoconductive layer in the light areas of the original image. Therefore, the positive charge on the surface of the insulating layer 3 in these areas is not reduced much, and the surface potential is almost kept constant as shown in characteristic V in FIG. 5. In the dark areas of the original image, irradiation thereof with light was not carried out in the preceding process, and photoconductive layer 2 presented high resistivity. In this process light is applied thereto and therefore layer 2 resistivity is abnuptly lowered, and it becomes conductive, and the negative charge bound in the internal portion thereof is mostly discharged into conductive base layer 1, and only slightly bound by means of the positive charge on the surface of insulating layer 3.

Thus, in this process, the field of the positive charge on the surface of insulating layer 3 which had been Working comparatively strongly in the direction of the negative charge bound on photoconductive layer 2, becomes an external field, and the surface potential of the insulating layer 3 is abruptly increased, and this characteristic V is presented, along with the time for irradiating light on the whole surface in FIG. 5.

When this uniform blanket light radiation is carried out, the surface potentials V V of the insulating layer 3 respectively become V D and the surface potential of the dark areas of the original image becomes higher than the surface potential of the light areas of the original image, i.e. the potential is reversed from the preceding process and simultaneously the surface potential difference is increased.

By irradiating the whole surface and by taking into consideration various conditions such as photosensitive plate characteristics and times for prior charging, it is possible to form an electrostatic image having large contrast on the surface of insulating layer 3.

In electrostatic image formation in accordance with the present invention, equilibrium is kept between the charge bound on the photoconductive layer and that on the surface of the insulating layer, and then alternating corona discharge is applied to the surface of the insulating layer simultaneously with original image irradiation, and by means of the mutual effect of the both, a surface potential difference is formed on the surface of the insulating layer, and further, light is irradiated on the whole surface of the insulating layer and an electrostatic image is formed in accordance with the original image light-and-dark pattern, and therefore, when compared with conventional electrophotography, it is possible to obtain an electrostatic image of high contrast, having large surface potential difference and strong external field, and at the same time sensitivity is remarkably increased.

Next the electrostatic image formed on the surface of the insulating layer is developed by means of cascade development, magnetic brush development, powder cloud development, or such like conventional developing methods, by using developer mainly composed of charged color particles, and as is shown in FIG. 6, visible image 11 is obtained. At this time, the electrostatic image formed on the surface of the insulating layer is of high electrostatic contrast when compared with that of the Carlson process, and therefore it is preferable, in the case of cascade method, to use heavier carrier, in particular, the heavier carrier obtained by coating the surface of metallic or nonmetallic particles, whose granularity is more than 0.3 mg., with resin uniformly containing an electrostatic charge controlling agent.

When magnetic brush development is employed, in order to prevent the discharge of the surface charge of the highly insulating layer through the carrier, good results can be obtained when iron filings coated with resin are used.

When liquid development is employed, a mixture of halogenated hydrocarbons such as Freon or the like, dimethyl polysiloxane (silicon oil), or such like highly insulating oils dispersed with pigments or dyes can be effectively used.

If any of the developing methods should be employed upon an electrostatic image formed as above described, having an electrostatic pattern of negative or positive polarity and remarkably high contrast, it is possible to obtain a visible image of high density.

Next, visible image 11, as shown in FIG. 7, is transferred on copying material 13 such as a paper by means of the method according to which corona discharge, bias voltage, or like external voltage 12 is applied as is described in U.S. Pat. No. 2,637,651. Alternatively usable is the method according to which copying paper, having electrostatic capacitance larger than the electrostatic capacitance of the photoconductive material of the photosensitive plate, is closely adhered on the surface on which the electrostatic image is formed. A further alternative is the method according to which after development, the visible image and insulating layer are charged, and thereafter the developed image is transferred. Finally, the electrophotographic image can be obtained by fixing the transferred image through radiation of heat rays by means of infrared radiation, or the like.

In order to enable repetitive use of the photosensitive plate, the photosensitive plate is subjected to cleaning after the completion of transferring of the image by means of conventional cleaning methods, such as the fur brush method, or by a direct rubbing method wherein an elastic body is used to remove charged particles remaining on the surface of the photosensitive plate.

Beyond such plate cleaning, other cleaning should be preferably carried out to remove the electrostatic image charge on the photosensitive plate to increase the cleaning elfect, and for this purpose, before carrying out cleaning by particle removal, an alternating current corona discharge is applied onto the insulating layer, and the electrostatic image charge is thereby removed. Then a fur brush or the like is employed as above described and excellent results can be obtained. In this case an opposite polarity potential to that of the developer colored particles is applied to the fur brush and thereby the effect of cleaning can be remarkably increased, and in this case, it is possible to simultaneously provide the primary charge.

The elfect of this cleaning depends on the properties of the material of the insulating layer, particularly its adhesive characteristics, and therefore, the resins mentioned above are preferably used as the electrophotographic image forming materials. Among all these materials, fluorine resin film has excellent non-adhesive characteristics, and during cleaning, it accelerates the separation of developer colored particles, and remarkable cleaning can be attained, and in this respect it is most effective.

In the electrostatic image forming process of the present invention, the thickness of the translucent insulating layer 3 affects the quality of the electrostatic image along with the photoconductive layer. In particular, it affects sensitivity, contrast, and durability of the photosensitive plate, which are important factors, and in order to form an excellent electrostatic image and in order to use the photosensitive plate repeatedly for a long period of time, it is necessary that the thickness of the translucent insulating layer be within the range of 10 to 50 Examples for forming the electrostatic image in accordance with the present invention are as follows:

EXAMPLE I 10 g. of vinyl chloride was added to g. of cadmium sulfide activated by copper, and a little amount of thinner was added thereto, and the obtained mixture of photosensitive substance was coated by spraying same on an aluminum plate of thickness about 1 mm., so as to have the mixture of photosensitive substance become about 100,11. thick. Then, on the surface of photoconductive film, a film of Mylar of thickness about 15, was adhered by using an adhesive. A corona discharge of 6 kv. was irradiated on the surface of the Mylar layer and a positive charge was uniformly bound. Then the original image was irradiated for about 0.1-0.3 second on the abovementioned surface by means of a 10 lux tungsten lamp and at the same time, an alternating current corona discharge of AC 6 kv. was applied thereto. Thereafter, the whole surface was uniformly illuminated by means of a tungsten lamp for 1 to 2 seconds and an electrostatic image was formed in accordance with the light-and-dark pattern of the original image. Then the electrostatic image was developed by means of the magnet brush method, and thereby a visible image of remarkably excellent quality having high image density was obtained.

EXAMPLE II FIG. 8 shows a copying machine embodying a process of the invention. Photosensitive plate A comprising conductive base 12, photoconductive layer 2t and insulating layer 3! is set around the periphery of drum 12t rotating in the direction of the arrow FIG. 8. Plate A is primary or firstly charged by corona discharger 4t, and the thus charged insulating layer 3! is irradiated by an original image by lens 131 through discharger 8t and at the same time subjected to A.C. corona discharge to form thereon an electrostatic image, and thereafter the entire surface of the insulating layer is exposed to tungsten lamp .23t to form an electrostatic image according to dark-andlight pattern of the original image. Developer 14t includes magnet brush 152 for applying charged coloring particles to the electrostatic image to visualize it. The visualized image is transferred onto copying material 11t, which is moved in contact with the visualized image by transfer roller 16t, by applying corona discharge of polanty opposite to that of the charged particles through transfer corona discharger 10!.

Then the copying material 11t moves along the periphery of the hot fixer drum 181 having an infrared ray lamp 17t inside thereof, and the transferred image is thereby fixed, and finally an electrophotographic image is obtained on receiver 19t.

After such transfer, photosensitive plate A is cleared of charge forming the electrostatic image still remaining on the surface of insulating layer 3t by means of AC corona discharger 202, and then, at cleaner 212, the powder image remaining on insulating film surface 3! is cleaned by brushing same with rotary brush 22t having soft hair such as a fur brush on the periphery thereof, and the plate is thus prepared for repeated employment.

According to this device, primary charge is applied to the surface of the insulating layer of the photosensitive plate comprising a conductive support, a photoconductive layer and said insulating layer. An original image is then radiated, and at the same time AC corona discharge is applied to charge the surface of the insulating layer while keeping equilibrium with the charge induced on the photoconductive layer, and by means of the mutual effect of the two, the whole surface of the insulating layer is exposed to form the electrostatic image on the surface of the insulating layer, and the surface potential of the insulating layer is reversed, and an electrostatic image of the original image is formed. Therefore, it is possible to obtain an electrostatic image of surface potential having strong external charge field and increased sensitivity. The electrostatic image is formed on the insulating layer, and the development, transferring process and cleaning process are then carried out. By selecting the insulating layer of high resistivity and high resistance against wearing it is possible to prevent surface deterioration of the internal photoconductive layer and fatigue without deteriorating or damaging the surface thereof even if friction or pressure or such like physical effects are imparted thereto, and therefore, a photosensitive plate may be provided which can vw'thstand repeated use for a long period of time.

We claim:

1. A process for forming an electrostatic image in a photosensitive plate having a photoconductive layer exhibiting p-type or n-type semiconductivity and an overlying insulative layer comprising the steps of:

( l) applying a charge of one polarity to said insulative layer, and then (2) while exposing said photoconductive layer to a pattern of image radiation applying an alternating current discharge to said insulative layer.

2. A process for forming an electrostatic image in a photosensitive plate having a photoconductive layer exhibiting p-type or n-type semiconductivity and an overlying insulative layer comprising the steps of:

(1) applying a charge of one polarity to said insulative layer, and then (2) while exposing said photoconductive layer to a pattern of image radiation applying an alternating current discharge to said insulative layer, and then (3) exposing said photoconductive layer to blanket radiation.

3. A process for forming an electrostatic image on a photosensitive plate having a conductive base, a photoconductive layer overlying said base and exhibiting p-type or n-type semiconductivity and an insulative layer overlying said photoconductive layer, said photosensitive plate being characterized in having carrier charge of a polarity corresponding to the conductivity type of said photoconductive layer injectable from said conductive base into said photoconductive layer and bound in the region of the interface between said insulative and photoconductive layers, said process comprising the steps of:

(a) applying a first charge of a polarity opposite to the conductivity type of said photoconductive layer substantially uniformly onto said insulative layer to inject 12 and bind carrier charge in the region of the interface between said insulative and photoconductive layers,

(b) then exposing said photoconductive layer to a pattern of image light while applying an alternating current corona discharge onto said insulative surface, and then (0) exposing said photoconductive layer to activating light to discharge bound carrier charge remaining in the region of said interface and form a high contrast electrostatic image.

4. A process according to claim 3, wherein said insulative layer is transparent to both said image light and said activating light, and said photoconductive layer is exposed to said image light and said activating light through said transparent insulative layer.

5. A process for forming an electrophotographic image comprising the electrostatic image forming process claimed in claim 3 and the further terminal steps of:

(d) applying a developer to said photosensitive plate to visualize the electrostatic latent image on said photosensitive plate,

(e) transferring said visualized image onto a transfer member,

(f) fixing said visualized image on said transfer memher, and

(g) cleaning the surface of said photosensitive plate to remove residual developer and enable repetitive use of said photosensitive plate.

6. A process according to claim 5, wherein said developer is a liquid developer and step (d) is carried out by bringing said liquid developer into contact with said photosensitive plate.

7. An apparatus for forming an electrostatic image comprising:

(a) a photosensitive plate having a photoconductive layer exhibiting p-type or n-type semiconductivity and an overlying insulative layer,

(b) means for applying a charge of one polarity to said insulative layer,

(c) means for exposing said photoconductive layer to a pattern of image radiation while applying an alternating current discharge to said insulative layer, and

(d) means for exposing said photoconductive layer to blanket radiation.

8. An apparatus for forming an electrostatic image comprising:

(a) a photosensitive plate having a conductive base, a

photoconductive layer overlying said base and exhibiting p-type or n-type semiconductivity and an insulative layer overlying said photoconductive layer, said photosensitive plate being characterized in having carrier charge of a polarity corresponding to the conductivity type of said photoconductive layer injectable from said conductive base into said photoconductive layer and bound in the region of the interface between said insulative and photoconductive layers,

(b) charging means for applying a first charge of a polarity opposite to the conductivity type of said photoconductive layer substantially uniformly onto said insulative layer to inject and bind carrier charge in the region of the interface between said insulative and photoconductive layers,

(c) means for exposing said photoconductive layer to a pattern of image light while applying an alternating current corona discharge onto said insulative layer, and

(d) means for exposing said photoconductive layer with activating light to discharge bound carrier charge remaining in the region of said interface and form a high contrast electrostatic image.

9. An apparatus according to claim 8, wherein said insulative layer is transparent to both said image light and said activating light, and wherein means (c) and (d) in- 11. An apparatus according to claim 10, wherein said clude means for exposing said photoconductive layer developing means includes a liquid developer. through said transparent insulative layer.

10. An apparatus for forming an electrophotographic References Cited image comprising the apparatus for forming an electro- 5 UNITED STATES PATENTS static image as in claim 8 and further comprising:

(e? developing means for visualizing said electrostatic gg Image 3,457,070 7/1969 Watanabe et a1. 96-1 R X (f) mean for transferring said visualized unage onto a 3 536 483 10/1970 Watanabe et a1 R copy sheet, 10

(g) fixing mle1ants f0:l fixing said visualized image on JOHN M -HORAN, Primary Examiner sai copy s cc an (h) cleaning means for removing from said photosensi- HUTCHISON Asslstant Exammer tive plate residual developer remaining after the trans- U S c1 X R fer of the visualized image whereby said photosensi- 15 tive plate is prepared for repeated use. 96-1; 355-10, 17 

